In an automatic power transmission for an automotive vehicle, the gear ratios in the forward and reverse drive ranges of the transmission are selected by selectively actuating fluid operated frictional units including, for example, a high-and-reverse clutch, a forward drive clutch, a brake band and a low-and-reverse brake which are all operated by fluid pressure delivered from a hydraulic control system. In a known power transmission using these frictional units, the forward drive clutch in particular is maintained coupled throughout the conditions in which an automatic forward drive range is established in the transmission. When forward drive clutch alone is permitted to be operative, the first or "low" gear ratio in the automatic forward drive range is established in the transmission mechanism. If the brake band is applied with the forward drive clutch maintained in the coupled condition, then a shift is made in the transmission mechanism from the first gear ratio to the second gear ratio in the automatic forward drive range. If, furthermore, the brake band is released and the high-and-reverse clutch in turn is made operative with the forward drive clutch held coupled, there is produced in the transmission mechanism an upshift from the second gear ratio to the third or "high" gear ratio in the automatic forward drive range. When, on the other hand, both of the high-and-reverse clutch and the low-and-reverse brake are made operative, a reverse drive gear position is obtained in the transmission mechanism. The high-and-reverse clutch is thus put into operation when either the third gear ratio in the automatic forward drive range or the reverse drive gear position is to be selected in the transmission mechanism.
The high-and-reverse clutch being utilized for making shifts to both of the third forward drive gear ratio and the reverse drive gear position, the clutch is usually designed to have a torque transmitting capacity compatible with the amount of load to which the clutch is subjected when the reverse drive gear position is to be selected in the transmission mechanism. The torque transmitting capacity of the high-and-reverse clutch is, for this reason, excessive when the third gear ratio in the automatic forward drive range is selected to provide the lowest speed reduction ratio between the driving and driven members of the transmission mechanism. In a known automatic power transmission, the torque transmitting capacity of the high-and-reverse clutch amounts to about 2.6 times the capacity which the clutch is required to have when the clutch is actuated to select the third forward drive gear ratio. If, therefore, the high-and-reverse clutch is actuated by a fluid pressure ordinarily used for actuating the other frictional units of the transmission mechanism, it is inevitable that a violent mechanical shock is created during shifting from the second gear ratio to the third gear ratio in the automatic forward drive range of the transmission.
With a view to reducing such a shock to be produced during upshifting between the second and third forward drive gear ratios, it has conventionally been proposed and put into practice to provide in the high-and-reverse clutch of an automatic power transmission of the described nature a combination of two pistons having different pressure acting areas. When shift is to be made from the neutral gear ratio to the reverse drive gear ratio in the transmission mechanism of such a transmission, a fluid pressure is applied to the piston having the larger pressure acting area so that the high-and-reverse clutch is actuated with a force sufficient for the reverse drive operation. When the clutch is to be actuated to make a shift to the third gear ratio in the automatic forward drive range, a fluid pressure is applied to the piston having the smaller pressure acting area so as to avoid production of a violent shock in the transmission mechanism. The expedient resorting to the use of such a double piston arrangement makes the clutch construction disproportionately complicate and is responsible for the high production costs of the clutch and accordingly the transmission system as a whole.